Casting operations

This document details the casting operations supported by TFHE-rs.

You can cast between integer types using either the cast_from associated function or the cast_into method.

The following example shows how to perform casting operations:

use tfhe::prelude::*;
use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheInt16, FheUint8, FheUint32, FheUint16};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let config = ConfigBuilder::default().build();
    let (client_key, server_key) = generate_keys(config);

    // Casting requires server_key to set
    // (encryptions/decryptions do not need server_key to be set)
    set_server_key(server_key);

    {
        let clear = 12_837u16;
        let a = FheUint16::encrypt(clear, &client_key);

        // Downcasting
        let a: FheUint8 = a.cast_into();
        let da: u8 = a.decrypt(&client_key);
        assert_eq!(da, clear as u8);

        // Upcasting
        let a: FheUint32 = a.cast_into();
        let da: u32 = a.decrypt(&client_key);
        assert_eq!(da, (clear as u8) as u32);
    }

    {
        let clear = 12_837u16;
        let a = FheUint16::encrypt(clear, &client_key);

        // Upcasting
        let a = FheUint32::cast_from(a);
        let da: u32 = a.decrypt(&client_key);
        assert_eq!(da, clear as u32);

        // Downcasting
        let a = FheUint8::cast_from(a);
        let da: u8 = a.decrypt(&client_key);
        assert_eq!(da, (clear as u32) as u8);
    }

    {
        let clear = 12_837i16;
        let a = FheInt16::encrypt(clear, &client_key);

        // Casting from FheInt16 to FheUint16
        let a = FheUint16::cast_from(a);
        let da: u16 = a.decrypt(&client_key);
        assert_eq!(da, clear as u16);
    }

    Ok(())
}